Chi-Square Local Independence Meets the Rasch Model

Hambleton et al. (1991) suggest using a chi-square test to identify local independence between two items. The procedure consists of constructing a 2x2 table for two items using the correct and incorrect answers of persons at the same level of ability (i.e., with the same raw scores on the test):

  Item X  
Item Y Right Wrong Total
Right
Wrong
A
C
B
D
YR = A+B
N-YR= C+D
Total XR = A+C N-XR = B+D N = A+B+C+D

This yields a chi-square statistic with 1 degree of freedom (without Yates' continuity correction, but the principle is the same):

This is minimized to zero, and local independence is apparently assured in this dataset at this ability level for this pair of items, when

AD = BC

i.e., when

A/N = XR/N * YR/N

as it would be when the data exactly conform to the Rasch model, or any other model in which the two items are conditionally independent.

In conventional usage, however, when χ2 < 3.84, there is a presumption of local independence with 95% confidence. Then "local independence" is declared!

Let us conduct an experiment based on this convention. Let us constrain the possible outcomes so that, if a subject succeeds on item X, that subject cannot fail on item Y. So the items are always locally dependent. The data matrix becomes:

  Item X  
Item Y Right Wrong Total
Right
Wrong
A
0
B
D
YR = A+B
N-YR= D
Total XR = A N-XR = B+D N = A+B+D

The chi-square test apparently becomes:

Then we can determine the relationship between successes on items X and Y for any value of chi-square, say N*k, where k is a constant. Accordingly,

Rewriting this in terms of probabilities, where PX is the probability of success on item X under these conditions, etc., and taking logarithms, such that K=loge(k):

It is seen that the relationship between the items is expressed in terms of log-odds difficulties, in accord with the Rasch model, despite that fact that the data do not accord with Rasch model conditions (because the items are locally dependent). Item Y is always easier than item X.

Further, for any particular chi-square "significance" value, e.g., 3.84, items with paired log-odds difficulties differing by less than loge (3.84 / N) might be unwittingly declared "locally independent".

Here is an example:

  Item X  
Item Y Right Wrong Total
Right
Wrong
20
0
67
13
87
13
Total 20 80 100

The log-odds difficulties of the two items are loge(13/87) = -1.90, and loge(80/20) = 1.39 logits, differing by -3.29. The criterion value is loge(3.84/100) = -3.26. Since -3.29 is less than -3.26, the items might be misleadingly declared "locally independent". For comparison, the chi-square computation above gives: χ² = 100 * 260^2 / (87 * 13* 20 * 80) = 3.74 with 1 d.f. one-sided → p=.053, so that the null hypothesis of local independence is not rejected.

Agustin Tristan

Hambleton R.K., Swaminathan H., Rogers H.J. (1991) "Fundamentals of item response theory", Sage publications Inc. London, Chapter 2, pp 9-12 and Exercise 6, pp 29-31

Chi-square local independence meets the Rasch model. Tristan A. … 16:1 p.861


Chi-square local independence meets the Rasch model. Tristan A. … Rasch Measurement Transactions, 2002, 16:1 p.861



Rasch Publications
Rasch Measurement Transactions (free, online) Rasch Measurement research papers (free, online) Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch Applying the Rasch Model 3rd. Ed., Bond & Fox Best Test Design, Wright & Stone
Rating Scale Analysis, Wright & Masters Introduction to Rasch Measurement, E. Smith & R. Smith Introduction to Many-Facet Rasch Measurement, Thomas Eckes Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr. Statistical Analyses for Language Testers, Rita Green
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Journal of Applied Measurement Rasch models for measurement, David Andrich Constructing Measures, Mark Wilson Rasch Analysis in the Human Sciences, Boone, Stave, Yale
in Spanish: Análisis de Rasch para todos, Agustín Tristán Mediciones, Posicionamientos y Diagnósticos Competitivos, Juan Ramón Oreja Rodríguez

To be emailed about new material on www.rasch.org
please enter your email address here:

I want to Subscribe: & click below
I want to Unsubscribe: & click below

Please set your SPAM filter to accept emails from Rasch.org

www.rasch.org welcomes your comments:

Your email address (if you want us to reply):

 

ForumRasch Measurement Forum to discuss any Rasch-related topic

Go to Top of Page
Go to index of all Rasch Measurement Transactions
AERA members: Join the Rasch Measurement SIG and receive the printed version of RMT
Some back issues of RMT are available as bound volumes
Subscribe to Journal of Applied Measurement

Go to Institute for Objective Measurement Home Page. The Rasch Measurement SIG (AERA) thanks the Institute for Objective Measurement for inviting the publication of Rasch Measurement Transactions on the Institute's website, www.rasch.org.

Coming Rasch-related Events
Sept. 27-29, 2017, Wed.-Fri. In-person workshop: Introductory Rasch Analysis using RUMM2030, Leeds, UK (M. Horton), Announcement
Oct. 13 - Nov. 10, 2017, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
Oct. 25-27, 2017, Wed.-Fri. In-person workshop: Applying the Rasch Model hands-on introductory workshop, Melbourne, Australia (T. Bond, B&FSteps), Announcement
Dec. 6-8, 2017, Wed.-Fri. In-person workshop: Introductory Rasch Analysis using RUMM2030, Leeds, UK (M. Horton), Announcement
Jan. 5 - Feb. 2, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
Jan. 10-16, 2018, Wed.-Tues. In-person workshop: Advanced Course in Rasch Measurement Theory and the application of RUMM2030, Perth, Australia (D. Andrich), Announcement
Jan. 17-19, 2018, Wed.-Fri. Rasch Conference: Seventh International Conference on Probabilistic Models for Measurement, Matilda Bay Club, Perth, Australia, Website
April 13-17, 2018, Fri.-Tues. AERA, New York, NY, www.aera.net
May 25 - June 22, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com
June 29 - July 27, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Further Topics (E. Smith, Winsteps), www.statistics.com
Aug. 10 - Sept. 7, 2018, Fri.-Fri. On-line workshop: Many-Facet Rasch Measurement (E. Smith, Facets), www.statistics.com
Oct. 12 - Nov. 9, 2018, Fri.-Fri. On-line workshop: Practical Rasch Measurement - Core Topics (E. Smith, Winsteps), www.statistics.com

 

The URL of this page is www.rasch.org/rmt/rmt161f.htm

Website: www.rasch.org/rmt/contents.htm